568 



SCIENCE. 



[N. S. Vol. XIV. No. 354. 



diaphragm will lie in the optical axis. 

 When the focusing tubes are removed the 

 lantern may be mounted on a stand by 

 means of a brass riug with set screws, and 

 then serves admirably for ophthalmoscopic 

 or laryngological work. 



The following lenses, diaphragms, etc., 

 are provided for the experiments in physio- 

 logical optics made by first-year students in 

 the Harvard Medical School. Convex lens, 

 four inches focus ; convex lens, twenty 

 inches focus ; concave lens, twenty inches 

 focus ; astigmatic cylinder, twenty inches 

 focus ; astigmatic cylinder, five and a half 

 inches focus ; mirror, convex on one side, 

 concave on other, one inch focus ; plain 

 mirror ; diaphragm with circular aperture 

 two millimeters in diameter; diaphragm 

 with vertical and horizontal slits ; dia- 



necessary. The light rays are very distinct 

 in any room after sunset. A hundred or 

 more students may work in the same lab- 

 oratory. If a focusing-cloth or other 

 screen be used, the observations may be 

 made in daylight. 



II. THE CIRCULATION SCHEME. 



The artificial scheme (Fig. 2) to illus- 

 trate the mechanics of the circulation in 

 the highest vertebrates consists of a pump, 

 a system of elastic tubes, and a peripheral 

 resistance. The inlet and outlet tubes of 

 the pump are furnished with valves that 

 permit a flow in one direction only. The 

 peripheral resistance is the friction which 

 the liquid undergoes in flowing through the 

 minute channels of a piece of bamboo. To 

 this must be added the slighter resistance 



Fig. 2. The circulating scheme. 



phragm with L-shaped slit ; clear-glass 

 slide ; ground-glass slide ; wooden screen 

 to serve as retina ; square bottle filled with 

 eosin solution, to study refraction. 



With this apparatus reflection and refrac- 

 tion from plain and spherical surfaces, 

 spherical and chromatic aberration, astig- 

 matism, myopia, and hypermetropia may 

 be studied. A special dark room is not 



due to friction in the rubber and glass 

 tubes. 



In this system the pump represents the 

 left ventricle ; the valves in the inlet and 

 outlet tubes the mitral and aortic valves, 

 respectively ; the resistance of the channels 

 in the bamboo the resistance of the small 

 arteries and capillaries. The tubes between 

 the pump and the resistance are the arte- 



